Heading Instability of Moored Ships in Long Waves

Heading Instability of Moored Ships in Long Waves

By: Krish P. Thiagarajan

Tanker vessels used for offshore oil production and storage are kept in station by turret mooring systems, enabling the vessels to weathervane in the direction of the dominant environmental loads. These passive weathervaning systems have been observed to be ineffective in swell-dominated long wave conditions. A notable feature of poor heading stability is the existence of a stable equilibrium at a large heading angle (50 – 60o) with respect to the direction of oncoming waves. Model experiments on a 1:60 scaled FPSO have confirmed this behavior, particularly when wave length is comparable to the ship length. A detailed study of the dynamics of the prototype FPSO was conducted using frequency domain (quasi-dynamic) and time domain (dynamic) analyses. The quasi- dynamic study shows the importance of the balance between the yaw drift moment and sway induced yaw drift moment on the final heading angle. A pitchfork bifurcation of the equilibrium is seen to occur when wavelength/ship length ratio equals 0.73. The time domain analysis provides data consistent with model test findings. The viscous damping of various modes on yaw motion is shown to be important and can significantly alter the heading instability. With lack of heading control, the ship motions, principally roll, can increase thus affecting onboard operations.